Balanced codes (encoded sequences) have gained a lot of interest, since they can be received using a simple AC (Alternate Current)-coupled interface, which is simpler and more compatible with major interface types than a DC (Direct Current)-coupled interface. A balanced code can be defined in a number of ways: a) a code in which the number of 0s and 1s within a length of any N consecutive bits is close to equal; b) a code in which the number of 1s and 0s within any length of M symbols is identical or close to identical; c) a code constructed such that the frequency spectrum resulting from the transmission of any code word has no DC component; or d) a code that has a finite digital sum variation. The use of “balanced code” in this application should be understood as including any of the definitions above.
The superposition of two encoded sequences over one channel has also gained of a lot of interest, as shown in U.S. Pat. Nos. 6,621,427, 6,351,501, 6,198,413, 6,628,213, 5,539,846 and 5,243,628. Usually, each encoded sequence carries different types of information. In such applications, the two superimposed encoded sequences generate a new code which needs to be balanced for reception with an AC-coupled interface.
Much of the existing communication network infrastructure, in particular in Ethernet Passive Optical Networks (EPON), uses low rate transmissions, for example 1 Gigabit per second (Gbps). We refer to such low rate transmission networks or communication systems as “old”. With the introduction of new high speed transmission systems such as 10 Gbps EPON, there is a need to continue using and to retrofit “old” systems in the simplest and most inexpensive way to carry the new high rate traffic. The use of known two code superposition schemes cannot answer this need, since such coding schemes are complex, change the original (low rate) coding and do not enable to detect only one of the encoded sequences.